(1) Field of the Invention
The invention relates to an airframe with a subfloor structure, for a rotary wing aircraft.
(2) Description of Related Art
The subfloor structure of a rotary wing aircraft is arranged within the lower part of the airframe between the cabin floor level and the outer loft of the rotary wing aircraft. Typically, a subfloor structure comprises a bottom shell, floor panels and a framework construction which connects to the floor and the bottom shell. For typical architectures, the framework comprises interconnected longerons and ribs as well as the lower portions of the main frames.
The longerons are generally planar longitudinal beam elements being basically arranged along the longitudinal axis of the aircraft and spanning straightforward upwards the entire length of the subfloor structure, whereas the ribs are generally planar crossbeams which are basically arranged orthogonally to the longitudinal aircraft axis and span straightforward upwards the whole subfloor width at their respective location. Both, the longerons and the ribs extend from the bottom shell to the floor plane and are basically flat stiffened webs with two caps arranged at both longitudinal borders of the web.
Hence, the framework adopts an orthogonal waffle grid configuration with numerous intersections. The flat webs of both the longerons and the ribs are basically perpendicular to the floor plane. The tasks of the subfloor structure are manifold. The subfloor structure takes on the one hand the payload loads and transmits them to the main frames.
On the other hand, the subfloor structure houses electrical and mechanical systems and the fuel tanks and further provides for substantial kinetic energy absorption in case of a crash scenario. The longerons confine, together with corresponding crossbeams, the fuel tank compartments in which elastomeric bladders are installed. Typically, lateral volumes enclosed between the longerons and curved side shells of the bottom shell are used for systems housing.
The document U.S. Pat. No. 6,959,894 discloses an impact resistant structure of a rotary wing aircraft, which includes: an energy absorber positioned under a floor of the rotary wing aircraft and directly connected to a cabin frame of the rotary wing aircraft. The energy absorber is arranged in accordance with a distribution of a ground reaction force on a general ground at a time of crash situation. Another aspect of the present invention provides an energy absorber, which includes: a plurality of independent hollow tubes of fiber reinforced composite material integrally formed by bundling only the hollow tubes. The hollow tubes are arranged so as to reduce a number of intersecting wall surfaces of the hollow tubes.
The document EP2455285 describes a shock absorbing structure for a helicopter. The shock absorbing structure is miniaturized by providing a beam-like member having a recess and a shock absorbing member. One end of the shock absorbing member is arranged in the inside of the recess and the other end of the shock absorbing member is arranged outside of the recess. The area of the recess overlaps the place where the structure member supports the structure even at a dead-stroke in which the shock absorbing member is bottomed out.
The document U.S. Pat. No. 6,427,945 discloses a subfloor structure of an aircraft airframe, particularly of a helicopter, with longitudinal beams and crossbeams that intersect each other and are interconnected to form a grid that is fixedly attached to a floor and a bottom skin of the aircraft fuselage. Structural elements such as pyramid frustums and reinforcements are arranged on the beams. The longitudinal beams and the crossbeams each have a trapezoidal cross-section that is preferably open on the wider base side, closed by a spine web along the narrow side, and bounded laterally by inclined leg webs that extend downwardly from the spine web at an angle outwardly relative to each other. The subfloor structure grid effectively absorbs the energy of a crash impact having both axial or vertical as well as non-axial or lateral impact force components.
Other prior art documents were perused with respect to the invention, i.e., the documents: “Composites soften impact” (in Structures by Rob Coppinger, page 26, London, 2005), DE3049429 and U.S. Pat. No. 3,652,050.